Add build rules for Bazel/Closure

[draffensperger]

Building with Bazel will enable local development that can change any part of the code in dependent packages and get quick incremental rebuilds.

Building with Closure will enable smaller JS bundles for use in <script> tags, and helps ensure that no Closure-incompatible changes would be introduced later on for users whose builds use Closure.

Relevant links:

• Bazel TypeScript rules
• Discussion for how to interop Bazel TS builds with Closure JS build rules

The NPM/WebPack builds would continue to be supported after this change.